The present invention pertains to printing inks. More particularly, the present invention pertains to printing inks used in ink jet applications. Even more particularly, the present invention pertains to additives used in ink jet printing formulations that provide heat stability and leveling.
As is known to those skilled in the art to which the present invention pertains, modern day, high speed printers use either drop-on demand (DOD) or continuous ink jet (CIJ) printing. Although both methods of printing are widely used today, they are very distinct technologies. In continuous ink jet printing, ink is supplied under pressure to a manifold region or area of a printer that distributes the ink to a plurality of orifices which are usually arranged in linear fashion. The ink discharges from the orifices in filaments which break into droplet streams. These droplet streams, then, have certain drops selectively charged and deflected from their normal trajectories. Printing or graphic reproduction is accomplished by depositing at least some of the selectively charged and deflected drops to a print receiving medium, while the other drops strike a drop catcher device. This technology is well known and well described in the literature. See, inter alia, U.S. Pat. No. 5,624,485.
The inks used in continuous ink jet printing are usually (lye-based and water-soluble. Such inks are developed with three criteria in mind, namely, water solubility, surface tension and viscosity.
The water solubility becomes extremely important with respect to the orifices and the manifold since the ink film on the printhead is allowed to dry when the printer is idle and must be leveled or resolubilized to reduce accumulation of ink on and in the orifices.
Ordinarily, continuous ink jet inks comprise a water-soluble polymer resin, a water-soluble dye as a colorant, a liquid carrier and water as a primary vehicle or carrier. Also, most dyes are blended with not only the above-noted ingredients, but other adjuvants including, for example, corrosion inhibitors, surfactants, biocides, humectants, etc., as well as mixtures thereof.
Today, many of the inks used in continuous ink jet printing also incorporate a polymeric fixing agent to impart waterfastness to the image formed on the substrate. One class of fixing agents is polyimine polymers based on N-hydroxy alkyl imine, wherein the alkyl portion is usually ethyl or propyl.
Ink jet inks may also utilize pigment-based colorants. As disclosed in U.S. Pat. No. 5,531,816 a pigment-based ink is formulated with at least one degglomerated pigment and a water-insoluble compound to reduce bleed and improve dry times. The use of large amounts of the water-insoluble compound necessitates formulation with at least one amphiphile.
Although current ink jet ink formulations offer waterfast inks that reduce bleed and have improved dry times, current ink jet ink frequently have misdirected jets presumably due to ink coagulation at the delivery orifice of the printhead. It has also been found, though, that the use of polyimine binders can further lead to problems with the jetting of inks due to misdirected jets.
It is hypothesized that jet malfunction is due to a plurality of causes including thickening of the ink as well as plugging of the orifice or encrustation of dried ink around the orifice causing the emerging ink stream to be misdirected. The art has therefore failed to suggest a water-based ink formulation that provides smooth film formation at the same time providing a continuous unrestricted flow of ink through the ink jet printhead orifice.